The field of the invention is that of electronic devices, or devices that comprise an electronic part, which must operate autonomously over an extended period of time without an external power supply, for much longer than would be permitted by the amount of power initially on board only.
Electronic devices of this kind may be for example devices with low consumption intended to be implanted in the human or animal body, for which it is impossible or complicated to provide an electrical connection for recharging. They may thus be for example measuring or detecting sensors with wireless communication, intended to be distributed or installed over wide areas, for example radio signal detectors, or environmental sensors in the natural environment, or any device for which considerable autonomy is required, for example devices intended to be worn as intelligent clothing, or on board in a system not intended to receive them, such as a surveillance or locating beacon.
It has been proposed to supply such a device, especially when a certain degree of miniaturization is sought, with electrical energy produced by a device forming a capacitor, the capacitance of which varies under the effect of mechanical energy received by the device itself. It may be for example a matter of recovering ambient mechanical energy, such as vibrations, variable mechanical deformations, variations in the pressure of a fluid, etc.
Such a manner of production converts mechanical energy to electrical energy by a transduction method called electrostatic transduction. From an electrical standpoint, the electrostatic transducer may be regarded as a variable capacitor. This manner of transduction passes through an electronic energy management circuit that can be miniaturized, but currently requires complex electronics for managing the charge and discharge cycles of the transducer.
Now, in the known devices, operation of such an energy management circuit requires several tens of volts and represents a sizable consumption, whereas the power levels generated by electrostatic devices are very low, typically in a range of power between a few nanowatts and a few hundreds of microwatts.
Among the first circuits proposed that operate at power levels of the order of the μW, a load-constrained conditioning circuit was proposed by Meninger et al. (2001) “Vibration-to-Electric Energy Conversion” in IEEE Transactions on Very Large Scale Integration (VLSI) Systems, Vol. 9, iss. 1, pp. 64-76, and a voltage-constrained conditioning circuit was proposed by Torres et al. (2006) 49th IEEE International Midwest Symposium on Circuits and Systems, MWSCAS '06, 6-9 Aug. 2006, pp. 65-69.
However, these two types of circuits are composed of switches synchronized with the mechanical movement of the transducer. The switches have to be controlled by an additional electronic circuit with high energy consumption compared to the power/energy generated. At present there is no viable implementation of this type of circuit for low powers, for example less than about a hundred microwatts.
As illustrated in FIG. 1, a circuit is proposed by Bernard C. Yen, “A Variable-Capacitance Vibration-to-Electric Energy Harvester”, IEEE Transactions on Circuits and Systems, Vol. 53(2), pp. 288-295, 2005.
This circuit utilizes the variations in capacitance of the variable capacitor Cvar by using a charge pump composed of diodes and capacitors. The electrical energy generated by the capacitance is first stored in the capacitance Cstore, and then sent periodically to Cres by a return circuit via the inductance Lfly.
This circuit has drawbacks, however. In particular, the return circuit has an inductance that causes limitations in terms of circuit miniaturization.
Moreover, it also comprises a switch, the control of which must be adjusted as a function of the operating conditions of the electrostatic transducer, for example the state of charge of the capacitors Cres and Cstore, as well as the amplitude and frequency of the vibrations, so as to maximize the power generated. This control is provided by an intelligent circuit, which also consumes power.
A purpose of the invention is to supply an electronic device capable of converting ambient mechanical energy to electrical energy for its supply, in its entirety or as a supplement or prolongation of autonomy. The invention also seeks to optimize the efficiency of this conversion, the simplicity, reliability and the cost of its manufacture and operation.